I have implemented a DAL using Rob Conery's spin on the repository pattern (from the MVC Storefront project) where I map database objects to domain objects using Linq and use Linq to SQL to actually get the data.
This is all working wonderfully giving me the full control over the shape of my domain objects that I want, but I have hit a problem with concurrency that I thought I'd ask about here. I have concurrency working but the solution feels like it might be wrong (just one of those gitchy feelings).
The basic pattern is:
private MyDataContext _datacontext
private Table _tasks;
public Repository(MyDataContext datacontext)
{
_dataContext = datacontext;
}
public void GetTasks()
{
_tasks = from t in _dataContext.Tasks;
return from t in _tasks
select new Domain.Task
{
Name = t.Name,
Id = t.TaskId,
Description = t.Description
};
}
public void SaveTask(Domain.Task task)
{
Task dbTask = null;
// Logic for new tasks omitted...
dbTask = (from t in _tasks
where t.TaskId == task.Id
select t).SingleOrDefault();
dbTask.Description = task.Description,
dbTask.Name = task.Name,
_dataContext.SubmitChanges();
}
So with that implementation I've lost concurrency tracking because of the mapping to the domain task. I get it back by storing the private Table which is my datacontext list of tasks at the time of getting the original task.
I then update the tasks from this stored Table and save what I've updated
This is working - I get change conflict exceptions raised when there are concurrency violations, just as I want.
However, it just screams to me that I've missed a trick.
Is there a better way of doing this?
I've looked at the .Attach method on the datacontext but that appears to require storing the original version in a similar way to what I'm already doing.
I also know that I could avoid all this by doing away with the domain objects and letting the Linq to SQL generated objects all the way up my stack - but I dislike that just as much as I dislike the way I'm handling concurrency.
I worked through this and found the following solution. It works in all the test cases I (and more importantly, my testers!) can think of.
I am using the .Attach() method on the datacontext, and a TimeStamp column. This works fine for the first time that you save a particular primary key back to the database but I found that the datacontext throws a System.Data.Linq.DuplicateKeyException "Cannot add an entity with a key that is already in use."
The work around for this I created was to add a dictionary that stored the item I attach the first time around and then every subsequent time I save I reuse that item.
Example code is below, I do wonder if I've missed any tricks - concurrency is pretty fundamental so the hoops I'm jumping through seem a little excessive.
Hopefully the below proves useful, or someone can point me towards a better implementation!
private Dictionary<int, Payment> _attachedPayments;
public void SavePayments(IList<Domain.Payment> payments)
{
Dictionary<Payment, Domain.Payment> savedPayments =
new Dictionary<Payment, Domain.Payment>();
// Items with a zero id are new
foreach (Domain.Payment p in payments.Where(p => p.PaymentId != 0))
{
// The list of attached payments that works around the linq datacontext
// duplicatekey exception
if (_attachedPayments.ContainsKey(p.PaymentId)) // Already attached
{
Payment dbPayment = _attachedPayments[p.PaymentId];
// Just a method that maps domain to datacontext types
MapDomainPaymentToDBPayment(p, dbPayment, false);
savedPayments.Add(dbPayment, p);
}
else // Attach this payment to the datacontext
{
Payment dbPayment = new Payment();
MapDomainPaymentToDBPayment(p, dbPayment, true);
_dataContext.Payments.Attach(dbPayment, true);
savedPayments.Add(dbPayment, p);
}
}
// There is some code snipped but this is just brand new payments
foreach (var payment in newPayments)
{
Domain.Payment payment1 = payment;
Payment newPayment = new Payment();
MapDomainPaymentToDBPayment(payment1, newPayment, false);
_dataContext.Payments.InsertOnSubmit(newPayment);
savedPayments.Add(newPayment, payment);
}
try
{
_dataContext.SubmitChanges();
// Grab the Timestamp into the domain object
foreach (Payment p in savedPayments.Keys)
{
savedPayments[p].PaymentId = p.PaymentId;
savedPayments[p].Timestamp = p.Timestamp;
_attachedPayments[savedPayments[p].PaymentId] = p;
}
}
catch (ChangeConflictException ex)
{
foreach (ObjectChangeConflict occ in _dataContext.ChangeConflicts)
{
Payment entityInConflict = (Payment) occ.Object;
// Use the datacontext refresh so that I can display the new values
_dataContext.Refresh(RefreshMode.OverwriteCurrentValues, entityInConflict);
_attachedPayments[entityInConflict.PaymentId] = entityInConflict;
}
throw;
}
}
I would look at trying to utilise the .Attach method by passing the 'original' and 'updated' objects thus achieving true optimistic concurrency checking from LINQ2SQL. This IMO would be preferred to using version or datetime stamps either in the DBML objects or your Domain objects. I'm not sure how MVC allows for this idea of persisting the 'original' data however.. i've been trying to investigate the validation scaffolding in the hope that it's storing the 'original' data.. but i suspect that it is as only as good as the most recent post (and/or failed validation). So that idea may not work.
Another crazy idea i had was this: override the GetHashCode() for all of your domain objects where the hash represents the unique set of data for that object (minus the ID of course). Then, either manually or with a helper bury that hash in a hidden field in the HTML POST form and send it back to your service layer with your updated domain object - do the concurrency checking in your service layer or data layer (by comparing the original hash with a newly extracted domain object's hash) but be aware that you need to be checking for and raising concurrency exceptions yourself. It's nice to use the DMBL functions but the idea of abstracting away the data layer is so to not depend on the particular implementation's features etc. So having full control of the optimistic concurrency checking on your domain objects in your service layer (for example) seems like a good approach to me.
Related
I'm currently working on a project using AIR and Flex that uses a remote data source to persist data locally in a SQLite database. Currently, there's a lot of copy and paste code that I was trying to alleviate, so since we already use a DAO pattern with several common queries that get passed to it and a type that creates SQLStatement values, I figured I would simplify our codebase even more.
I applied the Adapter pattern to allow a wider range of possible database operations to be performed ([saveOrUpdate, find, findAll, remove] => [selectSingle, selectMultiple, insert, updateSingle, updateMultiple, deleteSingle, deleteMultiple]). I also applied the Strategy pattern to two aspects of the statement runner: the first time for what sort of aggregated type to return (either an Array of records or an ArrayCollection of records) for the selectMultiple function; the second time for creating or not creating historical records (ChangeObjects).
After applying these patterns and testing some refactored code, it worked perfectly with an existing SQLite database. I neglected to test its compatibility with the remote data source, since the saving mechanisms are used during that process as well. After refactoring and simplifying our code and nearing the end of the development cycle, I tested the download.
It would read data from the SQLite database, despite the fact that there was actually no data in it according to sqlite3.
I will give the related piece of code for this.
public class BaseDaoAdaptee {
private var returnStrategy: ReturnTypeStrategy;
private var trackingStrategy: TrackingStrategy;
private var creator: StatementCreator;
public function insert(queryTitle: String,
object: DaoAwareDTO,
parameters: Array,
mutator: Function,
handler: Function): void {
var statement: SQLStatement;
mutator = creator.validEmptyFunction(mutator);
handler = creator.validFault(handler);
statement = defaultStatement(queryTitle, parameters, handler);
statement.addEventListener(SQLEvent.RESULT,
trackingStrategy.onInserted(object, mutator), false, 0, true);
statement.execute();
}
}
The code for the TrackingStrategy implemented:
public class TrackedStrategy
implements TrackingStrategy {
public function onInserted(object: DaoAwareDTO,
callback: Function): Function {
return function (event: SQLEvent): void {
var change: Change,
id:Number = event.target.getResult().lastInsertRowID;
creator.logger.debug((event.target as SQLStatement).itemClass + ' (id # ' + id + ') inserted');
(object as Storeable).id = id;
change = new Creation(object);
change.register();
callback();
};
}
}
The logger reads that various database records were inserted, and when stopped on a breakpoint in the above lambda, "object" has all proper values. When running a Select statement in sqlite3, no records ever get returned.
Why would this happen?
Turns out an open transaction on a SQLConnection value was the cause. Got to love team projects. Commit or rollback your SQLConnection transactions!
I'm using STE and I want to enable change tracking for an object and its children. What I currently have to do now is something like this.
int id = 1;
using(CustomerEntities context = new CustomerEntities())
{
CustomerSection custSection = context.CustomerSections.Include("CustomerSections.Customers").SingleOrDefault(p => p.ID == id);
custSection.StartTracking();
foreach(Customer cust in custSection.Customers)
{
cust.StartTracking();
{
return custSection;
}
What I am looking for is a way to automatically enable change tracking for the child objects too, without having to loop through each one and explicitly tell it to start tracking changes.
Thanks in advance for any insight.
Most probably you are using Self Tracking entities in combination with WCF. Then it's not needed to enable the changetracking manually. this is already done for you. The T4 template that generates the STE's includes a method decorated with the [OnDeserialized] attribute which starts the tracking once entities are deserialized (which occurs normally after reaching the client and converted back into runtime class instances fromout the xml that WCF generated for the transport. See the exact code example:
[OnDeserialized]
public void OnDeserializedMethod(StreamingContext context)
{
IsDeserializing = false;
ChangeTracker.ChangeTrackingEnabled = true;
}
Search your entities or the T4 template and you will find this soon.
Why this keep bugging me all day.
I have an entity with several references where i get from a context which I then Dispose.
Do some Changes and try to SubmitChanges(). While calling SubmitChanges() without .Attach() seems to simply do nothing. When using .Attach() I get the Exception :
An attempt has been made to Attach or Add an entity that is not new, perhaps having been loaded from another DataContext. This is not supported.
Any ideas?
L2S is very picky about updating an entity that came from a different DB context. In fact, you cannot do it unless you 'detach' it first from the context it came from. There are a couple different ways of detaching an entity. One of them is shown below. This code would be in your entity class.
public virtual void Detach()
{
PropertyChanging = null;
PropertyChanged = null;
}
In addition to this, you can also serialize your entity using WCF based serialization. Something like this:
object ICloneable.Clone()
{
var serializer = new DataContractSerializer(GetType());
using (var ms = new System.IO.MemoryStream())
{
serializer.WriteObject(ms, this);
ms.Position = 0;
return serializer.ReadObject(ms);
}
}
I want to have a collection of objects, which will be of a class I created called Server. A Server has a string property which is it's IP address, as well as many other pieces of data and objects.
I will have methods for adding and removing servers to this collection, and there will be a need to find a server by it's IP address occasionally. If I were doing this in C# I would use a Dictionary< where the IP string would be the key and the Server object would be the value. I could easily check to see if an item exists in the Dictionary before attempting to add it.
So my requirements are:
1. Ability to add items to the collection (I don't care where they go, front, back, middle)
2. Ability to remove items from anywhere in the collection.
3. Ability to determine if a particular IP address already exists in the collection.
4. Ability to get a reference to a Server object by it's IP.
Edit: Oh yes, I would like it to be strongly typed like the Vector... I guess it's not absolutely necesary, but would be nice.
So it seems like an associative arrays will give me what I need, except I'm not sure about how to do #3 or #4.
public var Servers:Object = new Object( );
public function AddServer(server:Server):void
{
//TODO:need to check if it exists first and throw an error if so
//(it's the caller's responsibility to call DoesServerExist first)
Servers[server.IP] = server;
}
public function RemoveServer(IP:string):void
{
//is it OK to attempt to delete an item if it doesn't already exist?
//do I need to check if it exists before doing delete?
delete Servers[IP];
}
public function DoesServerExist(IP:string):bool
{
//Do I loop through all the elements testing it's IP property?
//Or can I just do something like this?
if(Servers[IP] == null)
{
return false;
}
else
{
return true;
}
}
public function GetServer(IP:string):Server
{
return Servers[IP];//what is returned if this IP doesn't exist?
}
Call me goofy, but why not use the Dictionary class? That gets you everything except strong typing.
If you want strong typing then I'd say you need a custom container, which wraps up a Vector of Servers, and a Dictionary or associative array of IP strings that indexes into the Vector. Then you'd need to expose methods for access, test, insert and remove.
You can just use an array. Example:
var dict:Array = [];
var ip = "164.157.012.122"
dict[ip] = "Server name"
if (dict[ip] == "Server name"){
trace("Yay");
}
//membership
if (dict[ip]){
trace(ip + " is a member of dict");
} else {
trace (ip + " is not a member");
}
//removal:
dict[ip] = null;
AS3 does not really have a built in Dictionary class, unfortunately.
I've been spending some of my spare time working a set of collections for ActionScript 3 but I've hit a pretty serious roadblock thanks for the way ActionScript 3 handles equality checks inside Dictionary Objects.
When you compare a key in a dictionary, ActionScript uses the === operator to perform the comparison, this has a bit of a nasty side effect whereby only references to the same instance will resolve true and not objects of equality. Here's what I mean:
const jonny1 : Person = new Person("jonny", 26);
const jonny2 : Person = new Person("jonny", 26);
const table : Dictionary = new Dictionary();
table[jonny1] = "That's me";
trace(table[jonny1]) // traces: "That's me"
trace(table[jonny2]) // traces: undefined.
The way I am attempting to combat this is to provide an Equalizer interface which looks like this:
public interface Equalizer
{
function equals(object : Object) : Boolean;
}
This allows to to perform an instanceOf-esq. check whenever I need to perform an equality operation inside my collections (falling back on the === operator when the object doesn't implement Equalizer); however, this doesn't get around the fact that my underlying datastructure (the Dictionary Object) has no knowledge of this.
The way I am currently working around the issue is by iterating through all the keys in the dictionary and performing the equality check whenever I perform a containsKey() or get() operation - however, this pretty much defeats the entire point of a hashmap (cheap lookup operations).
If I am unable to continue using a Dictionary instance as the backing for map, how would I go about creating the hashes for unique object instances passed in as keys so I can still maintain equality?
How about you compute a hash code for your objects when you insert them, and then look them up by the hash code in your backing dictionary? The hashcode should compare === just fine. Of course, that would require you to have a Hashable interface for your object types instead of your Equalizer interface, so it isn't much less work than you are already doing, but you do get the cheap lookups.
How about rather doing this:
public interface Hashable {
function hash():String;
}
personally, I ask myself, why you want to do this ... hashing objects to obtain keys makes little sense if they are mutable ...
also, you might consider using a different approach, as for example this factory:
package {
public class Person {
/**
* don't use this!
* #private
*/
public function Person(name:String, age:int) {
if (!instantiationAllowed)
throw new Error("use Person.getPerson instead of constructor");
//...
}
private static var instantiationAllowed:Boolean = false;
private static var map:Object = {};
private static function create(name:String, age:int):Person {
instantiationAllowed = true;
var ret:Person = new Person(name, age);
instantiationAllowed = false;
}
public static function getPerson(name:String, age:int):Person {
var ageMap:Array = map[name];
if (ageMap == null) {
map[name] = ageMap = [];
return ageMap[age] = Person.create(name, age);
}
if (ageMap.hasOwnProperty(age))
return ageMap[age];
return ageMap[age] = Person.create(name, age);
}
}
}
it ensures, there's only one person with a given name and age (if that makes any sense) ...
Old thread I know, but still worth posting.
const jonny1 : Person = new Person("jonny", 26); const jonny2 : Person = new Person("jonny", 26);
is creating two completely different objects that will not compare using ==, guess I don't see why it's any more of a road block because of as3
The problem with AS3/JavaScript/EcmaScript is not that they create two different, equivalent objects.
The problem is that they cannot equate those two equivalent objects--only identity works, since there is no equals or hashCode methods that can be overriden with class-specific comparison logic.
For Map implementations such as dynamic Object or Dictionary, this means that you have to either use Strings or references as keys: you cannot recover objects from a map using different but equivalent objects.
To work around that problem, people either resort to strict toString implementations (for Object maps) which is undesirable, or to instance control for Dictionaries, as in #back2dos example, which introduces different problems (Also, note that #back2dos solution does not really guarantee unique Person instances since there is a time window during which asynchronous threads will be allowed to instantiate new Persons).
#A.Levy's solution is good except that in general, hashCodes are not strictly required to issue unique values (they are meant to map entries to buckets allowing for fast lookups, wherein fine-grained differentiation is done through equals method).
You need both a hashCode and an equals method, e.g.
public interface IEquable
{
function equals(object : Object) : Boolean;
function hash():String;
}
In any programming language,
const jonny1 : Person = new Person("jonny", 26);
const jonny2 : Person = new Person("jonny", 26);
is creating two completely different objects that will not compare using ==, guess I don't see why it's any more of a road block because of as3